This invention relates to a frame for a game racquet, and, more particularly, to a frame which is formed by filament winding.
The invention will be explained with reference to frames for tennis racquets. It will be understood, however, that the invention can be used to form frames for other game racquets, such as racquetball racquets, squash racquets, badminton racquets, etc.
Current racquet production methods take composite fiber and resin prepreg materials and roll, wrap, or fold the material by hand around an inflatable bladder and into a racquet preform shape, sometimes called a hairpin. A yoke which is formed by wrapping composite fiber and resin material around foamable resin material is attached to the hairpin to form the bottom of the hoop-shaped head portion of the frame. The yoke is attached to the hairpin by wrapping composite fiber and resin prepreg tape around the ends of the yoke and the hairpin. The preform is inserted into a mold having a cavity in the shape of the racquet frame, the bladder is inflated to force the hairpin against the surface of the mold cavity, and the mold is heated to cure the resin. The heat causes the foamable resin in the yoke to foam and expand, thereby forcing the yoke against the surface of the yoke portion of the mold cavity.
Some racquet frames have been formed by filament winding method in which a tow of resin wetted fiber or resin impregnated fiber (towpreg) is wound over a mandrel to form the preform. The preform is then inserted into a female tool and molded under heat while pressure is applied through an internal bladder inflated with compressed air.
Filament winding is a well known process for forming products from filament material such as graphite fiber, glass fiber, etc. In the filament winding process, filament material is wound around a rotating mandrel to form a tube. The filament material can be in the form of a tow, which is formed from a plurality of filaments or continuous fibers, for example, with 3000, 6000, or 12,000 carbon filaments per tow. Fewer or more filaments per tow can be used for carbon as well as other fibers, for example, from one to 50,000.
The filament material is coated with resin before or after winding. The wound tube is placed in a mold having the shape of the end product, and the mold is heated to cure the resin.
Filament wound parts are normally manufactured by winding discrete layers of material on a mandrel, using a helical or geodesic path. Each layer is made up of a 2-ply balanced laminate formed from a multicircuit winding pattern. The number of circuits required for each layer is calculated from the bandwidth of the material and the wind angle, so that the pattern closes and a constant thickness is achieved in the layer. Different layers may be wound at different angles, but each layer typically consists of a +xcex1xc2x0 angle ply and a xe2x88x92xcex1xc2x0 angle ply with respect to the longitudinal axis of the rotating mandrel, resulting from the reciprocating traverse of the carriage in the winding machine.
Tennis racquets have been produced by conventional filament winding. For example, EPO patent publication No. 0 470 896 describes forming a frame for a tennis racquet by winding fibers of glass, carbon, or other materials.
Preforms for racquet frames are generally made from fibers which have been wetted with resin. Sheets or ribbons of fiber and resin are referred to as prepreg. Tows of fiber and resin are referred to as towpreg.
The following definitions are used herein:
CIRCUIT: One complete traverse of the fiber feed mechanism of a filament winding machine.
TOW: Untwisted bundle of continuous filaments.
PLY: A single pass on a filament winding machine in which fiber is applied in one direction to the axis of the mandrel.
A LAYER is typically formed from a series of winding circuits resulting in a closed pattern of 2-plies; one of +xcex1xc2x0 and the other of xe2x88x92xcex1xc2x0.
LAMINA: A single ply or layer in a laminate made up of a series of layers.
The invention forms a racquet frame from composite material by using filament winding process to make a filament wound preform. The fibers are wetted by a resin before or after winding. A yoke which is made from fiber and resin prepreg wrapped around foamable resin is attached to the preform by wrapping fiber and resin prepreg tape around the ends of the yoke and the preform. Additional areas of the preform can be reinforced with fiber and resin prepreg tape as desired. The preform is cured in a mold under heat and pressure.